Layer system having an increased magnetoresistive effect and use of the same, wherein a first layer of an artificial antiferromagnet has a relatively low cobalt content

ABSTRACT

The layer system having an increased magnetoresistive effect contains at least one soft magnetic detection layer, a non-magnetic decoupling layer, which rests on the detection layer, and a layer partial system, which is located at a distance due to the decoupling layer, forms an artificial antiferromagnet, and which is decoupled from the detection layer. This partial system comprises a first ferromagnetic and a second ferromagnetic layer. The first ferromagnetic layer should be antiferromagnetically coupled (K 2 ) to the second ferromagnetic layer via a non-magnetic coupling layer. In addition, the side of the first ferromagnetic layer facing away from the coupling layer should be provided with an antiferromagnetic additional layer and be exchange-coupled (K 3 ) thereto and, in addition, should have a material composition that differs from the second ferromagnetic layer.

CROSS REFERENCE TO RELATED APPLICATIONS

This is the 35 USC 371 national stage of international applicationPCT/DE02/02898 filed on 7 Aug. 2002, which designated the United Statesof America.

FIELD OF THE INVENTION

The invention relates to a layer system having an increasedmagnetoresistive effect, comprising at least one detection layer madefrom a soft-magnetic material, at least one decoupling layer which bearsagainst the detection layer and is made from a nonmagnetic material, andat least one layer part-system which is spaced apart from the detectionlayer by the decoupling layer, forms an artificial antiferromagnet andis decoupled from the detection layer. This part-system includes atleast one second ferromagnetic layer, which adjoins the decouplinglayer, and at least one first ferromagnetic layer, which isantiferromagnetically coupled to the second ferromagnetic layer via acoupling layer made from nonmagnetic material and is provided, on itsside facing away from the coupling layer, with an antiferromagneticadditional layer, to which it is exchange-coupled. The invention alsorelates to possible uses of a layer system of this type.

BACKGROUND OF THE INVENTION

The use of corresponding layer systems has been proposed in particularfor measurement transducers, magneto couplers or current sensors. Alayer system of this type includes, as an important constituent, asubsystem or part-system which forms what is known as an artificialantiferromagnet (AAF for short). An AAF part-system of this type isadvantageous on account of a relatively high magnetic rigidity and a lowcoupling with respect to a magnetically softer detection or measurementlayer through what is known as the orange peel effect and/or throughmacroscopic magnetostatic coupling fields.

The structure of corresponding AAF part-systems is fundamentally known(cf. WO 94/15223 A). This system generally comprises at least twoferromagnetic layers, which are antiferromagnetically coupled via acoupling layer made from nonmagnetic material. It may, for example, beformed from two magnetic Co layers and one antiferromagnetic couplinglayer of Cu (cf. for example “IEEE Trans. Magn.”, Vol. 32, No. 5, Sep.1996, pages 4624 to 4626, or Vol. 34, No. 4, Jul. 1998, pages 1336 to1338, or “Journ. Magn. Magn. Mat.”, Vol. 165, 1997, pages 524 to 528).

To improve the magnetic rigidity of an AAF part-system of this type,that is to say its resistance to external outer magnetic fields, it isknown to arrange an antiferromagnetic additional layer on that layer ofthe part-system which is remote from the detection layer, referred tobelow as a first ferromagnetic layer. By means of this antiferromagneticadditional layer, the (first) ferromagnetic layer, which is thereforedirectly adjacent, is additionally pinned in its magnetization onaccount of the presence of an exchange coupling, so that overall the AAFpart-system becomes magnetically harder (known as exchange pinning orexchange biasing).

With a view to limiting the process costs involved in the production ofcorresponding layer systems and their AAF part-systems, it has hithertoalways been assumed that the two ferromagnetic layers of the AAFpart-system should consist of the same ferromagnetic material, forexample of Co or a Co alloy. In some cases, different layer thicknesseshave been planned for these two ferromagnetic layers, in order ifappropriate to allow or improve orientation of the magnetization. It hasbeen discovered that this restricts the magnetic matching of the AAFpart-system to the remaining parts of the layer system.

SUMMARY OF THE INVENTION

Therefore, it is an object of the present invention to configure thelayer system having the features described in the introduction in such away that greater flexibility with regard to the matching of its AAFpart-system becomes possible.

According to the invention, this object is achieved by the measuresgiven in claim 1. The starting point for this is the discovery that thiscan only be achieved by the first ferromagnetic layer, which is providedwith the antiferromagnetic additional layer and is exchange-coupled toit, having a different composition than the second ferromagnetic layer.Of course, the antiferromagnetic coupling between the two ferromagneticlayers must be maintained.

The advantages which are associated with the configuration of the layersystem in accordance with the invention reside in an increase in thedegree of freedom with regard to optimization of the AAF part-system.For example, in particular the following optimizations can be performedwith regard to

-   -   the indirect exchange coupling (what is known as the “RKKY”        coupling) between the two ferromagnetic layers via the        nonmagnetic coupling layer,    -   the direct antiferromagnetic exchange coupling between the        antiferromagnetic additional layer and the adjoining first        ferromagnetic layer,    -   the magnetic rotation properties (or the adjustability of the        magnetization direction) of the first ferromagnetic layer, and    -   what is known as the Néel coupling between the second        ferromagnetic layer and the soft-magnetic detection layer which        is decoupled therefrom.

Advantageous configurations of the layer system according to theinvention are given in the dependent claims.

For reasons of the magnetization properties of the part-system, thefirst ferromagnetic layer may particularly advantageously be selected tobe magnetically harder than the second ferromagnetic layer. This isbecause in a layer system according to the invention it is imperativefor the uppermost ferromagnetic layer, which faces the decoupling layer,i.e. in this case the second ferromagnetic layer, given an initialmagnetization (known as the “primary magnetization”), to have itsmagnetization reversed in an outer magnetic field, the process known asinitialization. This process is implemented by means of a 180° switchingoperation, in which domain walls are often involved. These walls canthen produce what are known as 360° walls in the switching ferromagneticlayer (=second ferromagnetic layer), which may have adverse effects onthe magnetoresistive effect. It has been discovered that, if amagnetically softer material is used for the upper, second ferromagneticlayer, it is possible to avoid the formation of undesired 360° walls ofthis type. In particular, the number of 360° walls (or the proportionthereof by area) decreases as a result.

Accordingly, the second ferromagnetic layer can consist of a CoFe alloywith in relative terms a lower Co content than the first ferromagneticlayer, so that it becomes magnetically softer.

The abovementioned magnetization reversal of the second ferromagneticlayer of the layer part-system is also promoted by the firstferromagnetic layer having a greater thickness than the secondferromagnetic layer.

The material used for the antiferromagnetic additional layer isadvantageously selected from the group consisting of NiO, FeMn, IrMn,NiMn, TbMn, CrPtMn, RhMn, PtMn and PdMn.

The coupling layer preferably consists of a material selected from thegroup of noble metals, in particular from the group consisting of Cu,Ag, Au, Pd and Ru.

The advantages of the configuration of the layer system in accordancewith the invention come to bear in particular if it is designed as anXMR system.

The layer system according to the invention may particularlyadvantageously be used in a magnetic field sensor, such as for example acurrent sensor, or in a magneto coupler.

Further advantageous configurations of the layer system according to theinvention will emerge from the claims which have not been discussedabove.

BRIEF DESCRIPTION OF THE DRAWING

For further explanation of the invention, reference will now be made tothe drawing, in which:

FIG. 1 diagrammatically depicts a layer system according to theinvention.

FIG. 2 shows a detailed view of an AAF part-system of this layer systemshown in FIG. 1.

DETAILED DESCRIPTION OF THE INVENTION

The layer system 2 according to the invention, which is shown in sectionin FIG. 1, is based on known multilayer systems which have an increasedmagnetoresistive effect ΔR/R. Accordingly, the magnetoresistive effectof this system is greater than that of known magnetoresistivesingle-layer systems with an anisotropic magnetoresistive effect (“AMR”effect) and in particular is over 2% at room temperature. The layersystem is either giant-magnetoresistive (“GMR”) ortunneling-magnetoresistive (“TMR”) or colossal-magnetoresistive (“CMR”)or has a giant magnetoimpedance or a giant AC resistance (“GMI”). Thedifferences between corresponding layer systems are explained, forexample, in the volume “XMR-Technologien”—Technologieanalyse:Magnetismus, Vol. 2—of the VDI Technology Center “Physi-kalischeTechnologien”, Düsseldorf (DE) 1997, pages 11 to 46. In this context,the term “XMR technologies” is used as a collective term for thetechnical know-how based on the magnetoresistance effects AMR, GMR, TMR,CMR and GMI. The layer system according to the invention is preferably aGMR or TMR system, in which case it has what is known as a “spin valve”structure.

To build up the layer system 2 according to the invention, first of alla buffer layer or a buffer layer system 4, as a base for a layerpart-system 5 to be deposited on, is provided on a substrate 3 in amanner which is known per se. This part-system should be a relativelyhard-magnetic part-system, which is also known as a reference layersystem. This part-system 5, which subsequently acts as an AAF layerpart-system (cf. the above-referenced WO 94/15223 A), is spaced apart,on its (upper) side facing away from the substrate 3, from asoft-magnetic layer 7 or a corresponding layer system by means of aninterlayer 6 made from a nonmagnetic material. In the case of a GMRlayer system, this interlayer may be made from a metallic material,while in the case of a TMR layer system it consists of an insulating orsemiconducting material. The soft-magnetic layer 7 is only weaklycoupled to the in relative terms magnetically harder layer part-system 5or is decoupled from this part-system. The interlayer 6 can therefore beconsidered as a decoupling layer. The figure indicates a possible orangepeel coupling and magnetostatic coupling by means of a curved arrow K1.The layer 6 ensures that the soft-magnetic layer 7 can have virtuallyany desired orientation of its magnetization. It can therefore be usedto perform the function of a detection layer or a measurement layer.Instead of a single detection layer of this type, it is also possible toprovide layer systems with a corresponding action, such as for examplesystems comprising two ferromagnetic layers or a system comprising oneferromagnetic layer, one nonmagnetic layer and one ferromagnetic layer(known as a “synthetic free layer system”).

As is intended to be indicated by a curly bracket in the figure, thestructure of the subsystem 8 formed from the part-system 5, the at leastone interlayer 6 located thereon and the at least one free magneticlayer 7 arranged thereon can be repeated periodically in a manner whichis known per se (cf. the abovementioned WO 94/15223 A). In general, thelayer system is also provided with a covering layer 9, which for reasonsof protection is intended to protect the layers below from oxidationprocesses. For TMR and GMR applications, in which a current flowsvertically through the layer stack, the covering layer must beelectrically conductive, for example must consist of Au or Cr, since itthen at the same time forms an upper electrode. For TMR applications, byway of example a lower electrode, for example in the form of a 30 nmthick Cu layer, is integrated in the buffer layer 4. The substrate 3 maybe an Si wafer or another surface of another structure, which canfundamentally be any desired structure, for example a structure used insemiconductor technology. Of course, the sequence of the layers in thelayer subsystem 8 with respect to the substrate 3 and its buffer layer 4may also be reversed.

The reference layer part-system or AAF part-system 5 which is to be usedfor the layer system 2 according to the invention shown in FIG. 1 can beseen in more detail from the section illustrated in FIG. 2. The buildingblocks for this layer part-system 5 are at least one second layer 12,which faces the decoupling layer 6, has a thickness t_(FM)′ and is madefrom a ferromagnetic material of saturation magnetization M_(FM)′, andat least one first layer 11, which has a thickness t_(FM) and is madefrom a ferromagnetic material of saturation magnetization M_(FM). Thesetwo ferromagnetic layers 11 and 12 are to be antiferromagneticallycoupled via a coupling layer 13 of thickness t_(K) made from anonmagnetic material, which is generally a noble metal, such as forexample Cu. In the figure, this coupling is intended to be indicated bya curved arrow K2. In addition, an antiferromagnetic additional layer 14of thickness t_(AFM), which is exchange-coupled to the adjoining layer11 in a manner known per se, is provided on that side of the firstferromagnetic layer 11 which faces away from the coupling layer 13. Thisexchange coupling is intended to be indicated in the figure by a curvedarrow K3. In general, the thickness t_(AFM) of the additionalantiferromagnetic layer 14 is significantly greater than that of thefirst ferromagnetic layer 11 which adjoins it, and is typically lessthan 30 nm, preferably less than 10 nm.

According to the invention, the first ferromagnetic layer 11 should havea different material composition than the second ferromagnetic layer 12.In this context, a material composition is to be understood as meaning adifferent elemental metal or a metal alloy which differs in terms of thealloying partners and/or the proportions of the alloying partners. Inaddition, the thicknesses t_(FM) and t_(FM)′ of the two ferromagneticlayers 11 and 12, respectively, may advantageously also differ, in whichcase the second ferromagnetic layer 12 preferably has a lower thicknesst_(FM)′ than the first ferromagnetic layer 11 of thickness t_(FM). Thethickness t_(FM) of the first ferromagnetic layer 11 and/or thethickness t_(FM)′ of the second ferromagnetic layer 12 are generally ineach case less than 5 nm, while the thickness t_(K) of the couplinglayer 13 is preferably less than 3 nm.

Suitable materials for the ferromagnetic layers 11 and 12 are, in aknown way, the three ferromagnetic elements Fe, Co or Ni or alloys whichcomprise or consist of these three elements. If appropriate, it is alsopossible for further elements, such as for example rare earths, to beadded to the alloy. Therefore, by suitably selecting the alloyingconstituents, it is possible to deliberately set the magnetic propertiesof the two ferromagnetic layers 11 and 12. By way of example, a CoFealloy with a Co content of over 60 atomic %, typically of 90 atomic %,can be used for the (second) layer 12, in order in this way to obtain analloy which is known to be relatively magnetically soft, with a lowmagnetostriction. By contrast, a CoFe alloy with in relative terms alower Co content than the second ferromagnetic layer 12 will then beselected for the (first) ferromagnetic layer 11, the Co content of thefirst ferromagnetic layer 11 preferably being less than 60 atomic %, forexample approximately 50 atomic %. Alloys of this type are known to beconsidered relatively magnetically hard.

The nonmagnetic coupling layer 13 generally consists of one of thenonmagnetic metals which are known for this purpose, in particular of anoble metal, such as Cu, Ag, Au, Pd or Ru, while a material selectedfrom the group consisting of NiO, FeMn, IrMn, NiMn, TbMn, CrPtMn, RhMn,PtMn, PdMn or FePt₃ is preferably selected for the antiferromagneticadditional layer 14.

An RKKY coupling between different alloys within the ferromagneticmaterials system Ni—Fe—Co was tested experimentally. It is known fromthese tests that Co-rich alloys lead to the highest values for thecoupling strength if Cu or Ru are selected as materials for the couplinglayer 13. In this context, it has also been established that thecoupling strength values are approximately equal, for example, for thealloys Co₉₀Fe₁₀ and Co₅₀Fe₅₀, whereas significantly lower values areobserved for Ni₈₀Fe₂₀.

To estimate the coupling strength, first of all a simple AAF part-systemwithout an exchange-coupled additional layer 14 will be considered. Inthis case, the RKKY coupling strength J_(RKKY), based on a saturationmagnetization M_(x) and a layer thickness t_(x) (with x in each casebeing equal to FM or FM′) of the corresponding ferromagnetic layer, canbe derived from the saturation field strength Hs using the followingrelationship:J _(RKKY) =H _(s)·(t _(FM) ·M _(FM) ·t _(FM) ′·M _(FM)′)/(t _(FM) ·M_(FM) +t _(FM) ′·M _(FM)′).

This coupling can readily be optimized by selecting the optimumcombinations of materials for layers 11 to 13.

On the other hand, an exchange-coupled additional layer has to be takeninto account in the layer system according to the invention. In thiscase, the coupling field H_(Exch) of the ferromagnetic layer 12 withrespect to the adjoining antiferromagnetic additional layer 14 isgenerally inversely proportional to the product of the thickness t_(FM)and the saturation magnetization M_(FM) of the ferromagnetic layer 11,so that the following relationship applies:H _(Exch) =J _(Exch)/(t _(FM) ·M _(FM)),where J_(Exch) denotes the coupling energy.

For this reason, an increase in the coupling field can be observed if achange is made to ferromagnetic layers of reduced thickness or with alower magnetic moment. Furthermore, it can be observed that a greatercoupling field results for the alloy Co₅₀Fe₅₀ than for the in relativeterms magnetically softer alloy Co₉₀Fe₁₀. Furthermore, magnetizationcurves for the double layer 14–11 (in this case with a ferromagneticlayer 11 made from Co₅₀Fe₅₀) reveal more abrupt switching orreorientation of the pinned additional layer 14. This can be recognizedfrom the square-wave shape of the hysteresis loop and is particularlypreferred in order to increase what is known as the magnetic window inangle sensor applications (=application area for the sensor in anexternal magnetic field). In this case, the magnetic window isdetermined from angle measurements, in which the reference layer (inthis case the upper ferromagnetic layer 12) begins to rotate in thefield direction, with the sensor signal being reduced by 5%.Consequently, an improvement in the square-wave form of the hysteresisloop leads to a correspondingly higher magnetic field window. Forexample, experiments carried out on the double layer system 14–11comprising IrMn—Co₉₀Fe₁₀ reveal a more extensive, S-shaped hysteresiscurve and consequently only allow use in a restricted window range.

The reorientation indicated by a double arrow M in FIG. 2 or acorresponding rotation of the magnetization in the upper secondferromagnetic layer 12 is critical in the case of what is known as aninverse AAF system, in which the direction of the net moment isdetermined by a thick lower (first) ferromagnetic layer 11. This conceptis of interest for an exchange-coupled AAF system, as is required forthe layer system according to the invention, since in this case theexchange coupling and the initial saturation of the AAF system can beinduced simultaneously, specifically at a high temperature and in astrong magnetic field. In this case, the temperatures are above what isknown as the blocking temperature of the antiferromagnetic material andabove the saturation field H_(S) of the AAF system. Furthermore, acorresponding inverse layer sequence can lead to a reduction andpossibly elimination of a Néel coupling in the case of relatively largeTMR sensors, where magnetostatic coupling at the edges is negligible, sothat second-order effects in the ideal sinusoidal sensor characteristicsare then correspondingly reduced.

In the case of a layer system with an inverse layer sequence, the upperferromagnetic layer has to rotate through 180° in terms of itsmagnetization after initial magnetization of the AAF part-system. Thesuccess of a rotation process of this nature is decisively influenced bythe magnetic properties of this layer. It can be improved by providing alayer comprising in relative terms a magnetically softer material and/ora layer having a predetermined uniaxial anisotropy. Very stable 360°domain walls can be formed in a thin layer of this type during therotation process. These domain walls are formed in the event of localvariations in the direction of the magnetocrystalline anisotropy betweengrains and are rather difficult to eliminate. They lead to adeterioration in the properties of the layer system, which can berecognized in particular from a reduced magnetoresistive effect. Afurther consequence is non-ideal switching properties of the freesoft-magnetic detection layer, caused by a strong stray field whichoriginates from the domain structure of an AAF system of this type. Forthis reason, magnetically softer alloys such as Co₉₀Fe₁₀ are a preferredchoice, whereas Co₅₀Fe₅₀ alloys are less suitable. Of course, it is alsopossible for other alloys from the Ni—Co—Fe materials system to beselected on the basis of this aspect. Amorphous NiFeCo alloys, whichhave a lower saturation magnetization and in which a uniaxial anisotropycan be established relatively easily in particular with the aid of alonger heat treatment step, are also particularly suitable.

The above considerations demonstrate that it is particularlyadvantageous for the second ferromagnetic layer 12 to consist of amaterial which is in relative terms magnetically softer than themagnetic hardness of the first ferromagnetic layer 11.

A corresponding specific layer system 2 according to the inventionhaving a structure or layer sequence as shown in FIGS. 1 and 2 thereforecomprises, on a substrate 3, for example made from an Si wafer, thefollowing successive layers:

-   -   a) a 3-layer system comprising Ta with a thickness of 5 nm, Cu        with a thickness of 30 nm and Ru with a thickness of 5 nm, as        buffer layer 4,    -   b) an IrMn layer with a thickness of 8 nm as an        antiferromagnetic additional layer 14,    -   c) a Co₅₀Fe₅₀ layer with a thickness t_(FM) of 2.5 nm as a first        ferromagnetic layer 11,    -   d) an Ru layer with a thickness t_(K) of 0.8 nm as a nonmagnetic        coupling layer 13,    -   e) a Co₉₀Fe₁₀ layer with a thickness t_(FM)′ of 1.5 nm as a        second ferromagnetic layer 12,    -   f) in the case of a GMR layer system, a Cu layer with a        thickness of 2.5 nm, and in the case of a TMR layer system an        AlO_(x) layer with a thickness of 1.5 nm, as a nonmagnetic        decoupling layer 6,    -   g) an Ni₈₀Fe₂₀ layer with a thickness of 6 nm or a        Co₉₀Fe₁₀/Ni₈₀Fe₂₀ double layer with a thickness of 1 nm and 4        nm, respectively, for the corresponding materials, as a free        soft-magnetic detection layer 7, and    -   h) a covering layer 9 made from Ta with a thickness of 5 nm.

1. A layer system (2) having an increased magnetoresistive effect,comprising at least one detection layer (7) made from a soft-magneticmaterial, at least one decoupling layer (6) which bears against thedetection layer (7) and is made from a nonmagnetic material, and atleast one layer part-system (5) which is spaced apart from the detectionlayer (7) by the decoupling layer (6), forms an artificialantiferromagnet, is decoupled from the detection layer (7) and a)includes at least one second ferromagnetic layer (12), which adjoins thedecoupling layer (6), and b) at least one first ferromagnetic layer(11), which b1) is antiferromagnetically coupled to the secondferromagnetic layer (12) via a coupling layer (13) made from nonmagneticmaterial, b2) is provided, on its side facing away from the couplinglayer (13), with an antiferromagnetic additional layer (14), to which itis exchange-coupled, b3) has a material composition which differs fromthe second ferromagnetic layer (12) b4) is magnetically harder than thesecond ferromagnetic layer (12); the first ferromagnetic layer (11)consists of a CoFe alloy with in relative terms a lower Co content thanthe second ferromagnetic layer (12); and wherein the Co content of thefirst ferromagnetic layer (11) is less than 60 atomic %.
 2. The layersystem as claimed in claim 1, characterized in that theantiferromagnetic additional layer (14) is made from a material selectedfrom the group consisting of NiO, FeMn, IrMn, NiMn, TbMn, CrPtMn, RhMn,PtMn, PdMn or FePt₃.
 3. The layer system as claimed in claim 1,characterized in that the thickness (t_(AFM)) of the antiferromagneticadditional layer (14) is less than 10 nm.
 4. The layer system as claimedin claim 1, characterized in that the first ferromagnetic layer (11) hasa greater thickness (t_(FM)) than the second ferromagnetic layer (12).5. The layer system as claimed in claim 1, characterized in that thethickness (t_(FM)) of the first ferromagnetic layer (11) and/or thethickness (t_(FM)′) of the second ferromagnetic layer (12) is/are ineach case less than 5 nm.
 6. The layer system as claimed in claim 1,characterized in that the antiferromagnetic additional layer (14) ismade from a material selected from the group consisting of NiO, FeMn,IrMn, NiMn, TbMn, CrPtMn, RhMn, PtMn, PdMn or FePt₃.
 7. The layer systemas claimed in claim 1, characterized in that the thickness (t_(AFM)) ofthe antiferromagnetic additional layer (14) is less than 30 nm.
 8. Thelayer system as claimed in claim 1, characterized in that the couplinglayer (13) is made from a material selected from the group of noblemetals, in particular from the group consisting of Cu, Ag, Au, Pd andRu.
 9. The layer system as claimed in claim 1, characterized in that thethickness (t_(K)) of the coupling layer (13) is less than 3 nm.
 10. Thelayer system as claimed in claim 1, characterized in that theantiferromagnetic additional layer (14) is made from a material selectedfrom the group consisting of NiO, FeMn, IrMn, NiMn, TbMn, CrPtMn, RhMn,PtMn, PdMn or FePt₃.
 11. The use of the layer system as claimed in claim1 in a magnetic field sensor or a magneto coupler.
 12. The layer systemas claimed in claim 1, characterized in that the first ferromagneticlayer (11) has a greater thickness (t_(FM)) than the secondferromagnetic layer (12).
 13. The layer system as claimed in claim 1,characterized in that the first ferromagnetic layer (11) has a greaterthickness (t_(FM)) than the second ferromagnetic layer (12).